1. Field of the Invention
The present invention relates to a moving image pickup apparatus and a still image recording method for the apparatus, a program for implementing the method, and a storage medium storing the program.
2. Description of the Related Art
The driving speed at which an image pickup apparatus, such as a video camera, is driven (hereinafter referred to as “the shutter speed”) is originally 1/60 seconds according to the NTSC standard, but recently, to expand the photographable range, a photographing mode in which the shutter speed is set to a lower value of 1/15 seconds, or an even lower value of ½ seconds comes into actual use.
On the other hand, a great majority of video cameras have an automatic focus adjusting device (autofocus device) employing a method of detecting the sharpness of an image based on an image signal obtained by photoelectrically converting an object image e.g. by an image pickup device, to set the same to an AF (autofocus) evaluation value, and controlling a focus lens position such that the AF evaluation value is maximized, to thereby achieve focus adjustment.
As the AF evaluation value, there is generally used a focusing voltage value of high-frequency components of an image signal extracted by a band-pass filter for passing a predetermined frequency range. In the case where a normal object image is shot, as shown in FIG. 10, the AF evaluation value increases as the image becomes more focused, and a focus lens position where the AF evaluation value is maximized is defined as an in-focus position.
However, as video cameras, there have been developed products which are capable of not only recording moving images in a magnetic tape but also recording still images in a memory. In such products, when a still image is recorded, control is provided such that the recording of the still image is inhibited until the AF control operation (focusing of the image) is completed.
In this case, there can be cases in which the AF control operation (focusing of the image) takes a long time, so that the recording of a still image is not permitted for a long time. Therefore, to avoid such an inconvenience, control is provided such that the recording of a still image is started when a predetermined time period (AF focusing waiting time period) has elapsed.
However, in a conventional video camera in which the shutter speed can be set low, a sampling period at which the AF evaluation value is sampled becomes longer when the shutter speed is set low, which results in a considerable increase in focusing time by the AF (autofocusing). For this reason, a video camera capable of having the shutter speed set low and recording still images suffers from the problem that focusing can hardly be achieved within the above-mentioned predetermined time period (AF focusing waiting time period).
As a solution to this problem, it can be envisaged that in a still image recording mode, the sampling period of the AF evaluation value is shortened by increasing the shutter speed only during an AF operation. However, setting a low shutter speed is originally intended to cope with shooting an object of an extremely low illuminant, and if the shutter speed is increased in shooting such a dark object, an image signal indicative of a sufficient contrast cannot be obtained, which makes it impossible to achieve focusing.